1. Field of the Invention
This invention relates to apparatus and method for determining the rate of flow of a fluid by measuring the electrical potential difference developed in the fluid as the fluid moves through a magnetic field.
2. Background Information
In many prior art in-line magnetic flow meters, an electrical potential difference developed in the fluid is generally sensed by a pair of electrodes contacting the liquid and spaced apart from each other by a voltage sensing path essentially equal to the diameter of a flow sensing passage. A magnetic field generated orthogonal to both the axis between the electrodes and the direction of flow through the sensing passage is provided by two coils of wire located on opposite sides of and outside of the passage.
It is well known in the flow measurement arts that the velocity of a small volume of a fluid flowing in a pipe varies with the radial position of the small volume. The maximum velocity is commonly found along the axis of a pipe and the minimum velocity along the walls of the pipe. Moreover, the flow profile commonly changes with flow rate for any particular selection of fluid, and operating temperature. Many flow sensing technologies, such as turbine meters, use sensors configured as probes that can be located a selected radial position within the flowing fluid. This position is selected to be one at which the flow signal varies linearly with the actual overall flow rate. This sort of selective sensor positioning is not available in prior art magnetic flow meters, partly because of a need to maintain the largest possible voltage sensing path in order to have a measurable flow signal.
In his U.S. Pat. No. 6,571,642, the entire disclosure of which is herein incorporated by reference, the inventor taught arrangements for increasing the measured signal voltage by increasing the length of the flow sensing path at the expense of an increase in pressure drop. As generally depicted in FIG. 1, these arrangements generally comprised providing a streamlined body extending along the axis of the sensing passage and supported by a vane extending inwardly from a wall of the flow sensing passage. These arrangements generally provided a quasi-annular voltage sensing path extending around the circumference of the flow passage between two electrodes located exposed on opposite sides of the support vane.